1. Field of the Invention
The present invention relates to an electrically conductive grounding strap which can drain or wick static electrical charge from the wearer. In particular, the invention relates to wrist straps and buckles made of an anti-static or static dissipative material.
2. Background
As reported in U.S. Pat. No. 4,577,256, static electricity provides problems in the electronics and other industries, particularly with the advent of integrated circuits and other microelectronic components. Components such as integrated circuits, for instance, may be disabled or destroyed by over-voltage or power density resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. Power densities resulting from excessive potential and imperfections in a silicon circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit""s performance. Yet a person walking on a carpet on a dry day can accumulate as much as 30,000 volts of potential, and he can triboelectrically generate thousands of volts by simply changing his position in his chair or handling a styrofoam cup.
Such a person can inadvertently discharge such static electric potential into a circuit or component by touching it and causing overvoltage or excessive power density. Additionally, the potential in such a person""s body can induce a charge in a circuit that can later cause overvoltage or excessive power density when the circuit is subsequently grounded. More and more frequently, therefore, personnel in industries in which integrated circuits and other microelectronic components are handled or assembled are taking measures to limit the failure rate of those circuits and components by attempting to keep both them and their environment at a zero electrical potential. Such measures include providing workers and work stations with antistatic carpet, conductive or dissipative grounded desk top work surfaces; hot air ion generators which emit ions to neutralize static charges; and grounding straps to keep workers at zero potential.
A person working on microelectronic components or integrated circuits may be completely unaware that he has accumulated minor static electrical discharges, and may therefore unknowingly be in a position to disable circuits on which he is working or which he is handling. If he is wearing a grounding strap which is not sufficiently effective, he may be unaware that electrical discharges transmitted from his fingers are disabling these circuits. A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts. No one may discover that the circuits have been disabled or damaged until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of these circuits once in the field is far costlier than avoiding potential failure while the wearer is handling the circuits. Thus, the wearer""s employer typically must depend upon the effectiveness of the wrist strap to maintain a lower failure rate of such electronic circuits and components, by maintaining continuous electrical contact with the wearer""s wrist.
Over the years, various types of grounding straps have been developed in an effort to maximize the functionality and comfort of the device. The straps may be made of fabric, elastic fabric, metal or metal expansion type bands. The bands may be adjustable or non-adjustable and available in different sizes. The bands can be slid on over the hand or may be fastened via snaps, velcro, buckles, etc. Straps have also been developed which contain a monitor for detecting electrical continuity between the band and the ground.
The term xe2x80x9cconductivexe2x80x9d herein, and according to its customary usage in the art, means an electrical resistance of between zero and 105 ohms. Similarly, xe2x80x9cdissipativexe2x80x9d means a resistance of between 105 and 1012 ohms and xe2x80x9cinsulativexe2x80x9d means a resistance of more than 1012 ohms.
A grounding strap must have several features in order to perform its grounding function effectively. First, it must ensure that the wearer""s skin is electrically connected to the ground. This connection is typically accomplished by a conductive surface on the inside of a strap contacting the skin. The conductive surface is electrically connected to a grounding cord which leads from the strap to a grounded electrical connection. The electrical resistance of the cord resistor is typically about 1 megaohm whereas the resistance of the conductive material on the interior of the strap is well below this value.
Second, the exteriors of any metal band or buckle used on a grounding strap must be encapsulated or coated to prevent electrical shock to the person wearing the bracelet and to prevent damage to the device being worked upon due to a too rapid static discharge. Typically, encapsulation is carried out by applying a plastic film molded from an insulative polymer, for example TEDLAR(copyright) from Dupont, or a paint containing an insulative material (i.e. an electrical resistance of greater than 1012 ohms). All prior art grounding strap inventions contain insulative exteriors. In fact, the regulatory agency of the static control industry states in the EOS/ESD S-1 that the outside of the grounding strap and buckle should be insulative.
However, the materials currently being used on the exterior of grounding straps and buckles are not conductive due to safety concerns for the wearer. Thus, the risk of accumulation of static electricity on the surface of the strap or buckle is a serious problem since this exterior electricity cannot be dissipated though electrical grounding and neutralization of surface static by ionization is not feasible or practical. Ideally, the electrical resistance of the exterior of the grounding strap should be between 106 and 1012 ohms and non tribogenerative. If the resistance is greater than 1012 ohms, the wearer might cause device damage if the static strap or buckle comes in close proximity to the device, especially if the strap or buckle contains static electricity on the surface due to tribogeneration. For example, TEDLAR(copyright) is an insulating polyvinylidenefluoride film laminate which has been used to encapsulate the exterior of metallic grounding bands. However, when rubbed against other materials, TEDLAR(copyright) will tribogenerate a static charge which may dissipate and cause static electric damage due to too rapid a discharge or secondary field generation. Further, if the electrical resistance of the exterior of the strap or buckle is less than 106 ohms, the wearer is at risk of bypassing the current limiting resistor and for receiving an electrical shock.
Devices at risk from static electricity are becoming more and more static sensitive. Thus, it is increasingly important and desirable to develop grounding straps which are more effective at dissipating static electrical charges.
See also: Antonevich et al., U.S. Pat. No. 4,475,142; Barrett, II et al., U.S. Pat. No. 4,676,561; Bertet et al., U.S. Pat. No. 4,375,713; Breidegam et al., U.S. Pat. No. 4,577,256; Breidegam et al. U.S. Pat. No. 4,639,825; Breidegam et al., U.S. Pat. No. 4,745,519; Breidegam et al., U.S. Pat. No. 4,782,425; Brosseau et al., U.S. Pat. No. 3,857,397; Burke et al., U.S. Pat. No. 3,596,134; Christansen et al., U.S. Pat. No. 4,398,277; Cohen et al., U.S. Pat. No. 4,596,053; Fiedler et al., U.S. Pat. No. 4,605,984; Freitag et al., U.S. Pat. No. 1,940,491; Gandelman et al., U.S. Pat. No. 3,685,106; Gordon et al., U.S. Pat. No. 4,662,695; Gordon et al., U.S. Pat. No. 4,755,144; Hee et al., U.S. Pat. No. 4,847,729; Hee et al., U.S. Pat. No. 4,878,148; Ho et al., U.S. Pat. No. 4,296,532; Mykkanen et al., U.S. Pat. No. 4,373,175; O""Neill et al., U.S. Pat. No. 2,588,655; Otten et al., U.S. Pat. No. 1,760,913; Rees et al., U.S. Pat. No. 4,654,748; Rieth et al., U.S. Pat. No. 4,096,688; Sands et al., U.S. Pat. No. 4,664,158; Swainbank et al., U.S. Pat. No. 4,402,560; Vandermark et al., U.S. Pat. No. 4,459,633; Weiss et al., U.S. Pat. No. 4,816,964; Weiss et al., U.S. Pat. No. 4,845,585; Weiss et al., U.S. Pat. No. 5,018,044; Weiss et al., U.S. Pat. No. 5,134,528; West et al., U.S. Pat. No. 5,568,351; Williams et al., U.S. Pat. No. 5,036,423.
The present invention provides a method and apparatus for removing static electricity from a person.
More particularly, the present invention provides a conductive body strap which may be attached to a ground so as to safely drain static electricity from the wearer. The strap has a conductive interior and an anti-static, static dissipative, or non-tribogenerative exterior. The anti-static exterior has an electrical resistance of between 106 and 1012 ohms which will not create static electricity when rubbed and will not wear off over time.
The straps optionally contain fasteners and both the strap and fastener may be fabricated from homogenous, coated or laminated materials. The straps may be composed of anti-static fabric or metal encapsulated in anti-static or static dissipative paint or plastic. The fasteners may be snaps, velcro or buckles composed of anti-static or static dissipative plastic or may be composed of metal encapsulated in anti-static or static dissipative paint or plastic.
In a preferred embodiment, the strap fits around the wrist and contains a buckle so as to adjust to fit multiple users. The strap is fabricated from non nickel containing stainless steel and is coated with an anti-static plastic overlay. The strap is secured with a plastic buckle that is composed entirely of an anti-static polymer. The exterior of the buckle contains a snap which can electrically connect the strap to a resistor fitted grounding cord through which the static electricity may be drained to ground.
Other aspects and embodiments of the invention are disclosed infra.